Method for correcting sensor signal

ABSTRACT

The invention provides a method of compensating for the error of a signal indicative of the mass of air flow output from an air mass flow sensor for an internal combustion engine, the method comprising deriving an air mass flow value from an air mass flow sensor output, calculating a compensated air mass flow value based on the air mass flow value and a compensation value, calculating an estimated air mass flow value from engine operating parameter values, calculating a new compensation value based upon the compensated air mass flow value and the estimated air mass flow value and using the new compensation value for subsequent calculations of the compensated air mass flow value.

TECHNICAL FIELD

The present invention relates to a method of correcting a sensor signal.More particularly, the invention is concerned with providing a method ofcompensating for calibration errors or “sensor drift” of a signal thatis output from a sensor for measuring the mass of airflow entering aninternal combustion engine.

BACKGROUND TO THE INVENTION

Manufacturers of internal combustion engines are under constant pressureto achieve reductions in the quantities of harmful gases andparticulates that are produced by the engine. At the same time, it isnecessary to maintain current levels of engine performance to whichconsumers have become accustomed. In order to satisfy theserequirements, it is essential both to monitor accurately and to controlprecisely the operational parameters of the engine.

The ability to measure accurately the mass of air flow that enters intothe engine is critical to the efficient operation of modern enginemanagement systems. Typically, an air mass flow (AMF) sensor is employedto perform this function.

Air mass flow sensors typically output a variable voltage that isproportional to the amount of air entering the engine cylinders via theair inlet of the engine. Some types of air mass flow sensors may alsooutput a variable frequency voltage based upon the mass of air flowingtherethrough. One approach taken to measure the mass of airflow is basedon the so-called “hot wire” principle, in which a voltage is applied toan electrically conductive element in the form of a wire. The wire isdisposed in an airflow sampling channel and is heated by an electricalcurrent that is induced by the applied voltage. As air flows across thewire, the temperature of the wire is reduced and control circuitry willact to increase the current flowing through the wire in order tomaintain a predetermined temperature. The air mass flow sensor outputs avoltage proportional to the current flow to an engine management system.The engine management system will derive a value of air mass flow fromthe received voltage value by reference to an air mass flow sensorcharacteristic.

A typical air mass flow sensor characteristic is shown in FIG. 1. Theordinate axis denotes the air mass flow in grams per second (g/s) andthe abscissa axis denotes the corresponding voltage (V) that is outputby the sensor. Line A represents the air mass flow sensor characteristicas calibrated and stored in the engine management system. It will beappreciated that the relationship between voltage and air mass flow isnon-linear.

A disadvantage of air mass flow sensors is that their characteristic issusceptible to drift over time. As a result, the voltage output from thesensor, which is input to the engine management system, will beinterpreted as an incorrect value of air mass flow. The problem isexacerbated in applications where the air mass flow sensor is requiredto measure accurately a wide range of flow rates, for example on largecapacity engines. It can be seen from FIG. 1 that for a specific value‘X’ of air mass flow, the air mass flow sensor will output graduallydecreasing values of voltage, V1, V2, and V3 respectively, as the timefrom the last sensor calibration increases, as shown by curves A, B andC, respectively.

The error-prone voltage signal that is output from the air mass flowsensor adversely affects the ability of the engine management system toregulate accurately other engine operating parameters, such as theair-fuel ratio, that are vital for efficient combustion. Therefore,critical operational factors such as engine fuel economy, engineperformance and exhaust emissions are likely to be influencedundesirably.

Although it is possible to re-calibrate the air mass flow sensorperiodically, this would be inconvenient and time consuming since itwould be necessary to present the vehicle to a re-calibration specialistto have the work carried out.

SUMMARY OF INVENTION

The invention is directed to addressing the problem described above and,from a first aspect, provides a method of compensating for thecalibration error of a signal indicative of the mass of air flow from anair mass flow sensor for an internal combustion engine. The methodcomprises deriving an air mass flow value from an air mass flow sensoroutput, calculating a compensated air mass flow value based on the airmass flow value and a compensation value, calculating an estimated airmass flow value from engine operating parameter values, calculating anew compensation value based upon the compensated air mass flow valueand the estimated air mass flow value, and using the new compensationvalue for subsequent calculations of the compensated air mass flowvalue.

Preferably, the step of deriving an air mass flow value includesconverting the air mass flow sensor output by means of an interpolateddata map, wherein the interpolated data map is defined by a plurality ofvoltage/flow pairs.

A benefit of the invention is that the value of air mass flow that ismeasured by the air mass flow sensor has a compensation value, orcorrection factor, applied to it constantly in order to compensate forthe drift in the air mass flow sensor. Moreover, the compensation valuesare updated periodically when certain enabling conditions are satisfied,as will be described herein, so as to adapt to the gradually increasingmagnitude of sensor drift.

In a preferred embodiment of the invention, each of the plurality ofvoltage/flow pairs are identified by a respective index value, eachindex value relating to a predetermined engine operating region. As aresult, the compensation value to apply to the measured value of airmass flow is selected from a plurality of compensation values, whereinthe compensation value is selected based upon the index that isidentified according to the current engine operating region. Thus, thenumber of compensation values correspond to the number of index values.

Preferably, the plurality of compensation values are stored in the formof a one-dimensional data map.

A preferred feature of the invention is that the step of calculating anew compensation value is triggered by a timer having a plurality ofpredetermined limits. The limits may be set depending on how frequentlyit is desired to update a particular compensation value that correspondsto a particular engine operating region. For example, preferably, themethod includes incrementing the timer to count the number of times itis verified that each one of a plurality of enabling conditions issatisfied. The step of triggering the calculation of a new compensationvalue may be initiated when the number of times exceeds thepredetermined limit for the associated engine operating region.

Advantageously, the timer limits are selected to be relatively high forfrequently encountered engine operating regions and relatively low forless frequently encountered engine operating regions. This ensures thatsystem resources are not burdened unnecessarily by attempting to updatecompensation values that relate to commonly encountered engine operatingregions more frequently than is necessary.

It is a preferred feature of the invention that the step of verifyingthe plurality of enabling conditions includes the step of comparing afirst stored fuel demand value with a second stored fuel demand valueand determining if the difference between the first and second storedfuel demand values is within a predetermined limit.

From a second aspect, the invention resides in an engine control systemfor use in an internal combustion engine, the engine control systemincluding a compensating unit for compensating for the error of a signalindicative of the mass of air flow from an air mass flow sensor for aninternal combustion engine, the compensating unit including a derivingunit for deriving an air mass flow value from an air mass flow sensoroutput, a summing unit for calculating a compensated air mass flow valuebased on the air mass flow value and a compensation value, a calculatingunit for calculating an estimated air mass flow value from engineoperating parameter values, and a learning unit for calculating a newcompensation value based upon the compensated air mass flow value andthe estimated air mass flow value and using the new compensation valuefor subsequent calculations of the compensated air mass flow value.

It will be appreciated that preferred features of the first aspect ofthe invention may be incorporated in the second aspect of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

In order for the invention to be more readily understood, reference willnow be made, by way of example only, to the accompanying drawings inwhich:

FIG. 1 represents a set of typical voltage/airflow characteristic graphsof an air mass flow sensor showing how the characteristic may changeover time;

FIG. 2 is a high-level functional block diagram of an embodiment of theinvention;

FIG. 3 is a more detailed functional block diagram of that shown in FIG.2;

FIG. 4 shows an exemplary sensor characteristic graph that shows valuesof air mass flow against voltage;

FIG. 5 shows an exemplary interpolated map representing the graph ofFIG. 4 together with a corresponding one-dimensional data map;

FIG. 6 shows a one-dimensional map corresponding to the one-dimensionalmap of FIG. 5 and depicts the derivation of index values;

FIG. 7 is a functional flow diagram that represents an embodiment of theinvention during the “search mode”;

FIG. 8 is a functional flow diagram that represents an embodiment of theinvention during the “learn mode”; and

FIG. 9 is a functional flow diagram that represents an embodiment of theinvention during the “calculation mode”.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, the term “engine speed” is used in theusual context and takes the units of revolutions per minute (rpm).“Engine load” is a synonym for “fuel demand” and takes the units of mgfuel. The term “operating point” is used to define the instantaneousoperating condition of the engine at any given time and the term“operating region” denotes a region of grouped engine operating points.

FIG. 2 shows a functional block diagram of an air mass flow sensorcompensation system 2 of the invention. The system 2 comprises an airmass flow sensor 4 that is arranged within the fresh air intake of acompression ignition internal combustion engine (not shown). The airmass flow sensor 4 is arranged to provide an input voltage to air massflow calculation means 6 which receives the voltage value supplied bythe air mass flow sensor 4 and calculates a corresponding value of airmass flow. The calculated value of air mass flow is then output by theair mass flow calculation means 6 to an engine management system 8 thatuses the received value of air mass flow for subsequent engine controlcalculations.

Referring to FIG. 3, the air mass flow calculation means 6 comprises asensor characteristic module 10, a learn compensation module 12 and alearning module 14.

The sensor characteristic module 10 receives a filtered voltage signalfrom the air mass flow sensor 4 and outputs a signal that is indicativeof the mass of air flowing through the air intake manifold. As has beenmentioned, the relationship between the mass of airflow and thecorresponding voltage value, or the so-called “sensor characteristic”,may be illustrated in a graph, such as that shown in FIG. 4.

FIG. 4 shows a sensor characteristic having an abscissa axis defined byoutput voltage values of the air mass flow sensor 4. The ordinate axisof the graph is defined by the mass of airflow that corresponds to aparticular sensor output voltage. Thus the graph illustrates how thevoltage output of the air mass flow sensor 4 changes according to themass of air flowing past the air mass flow sensor 4.

It will be appreciated that the graph of FIG. 4 is a continuous andnon-linear curve that is represented by an infinite number of points. Ingeneral, engine management systems do not utilise graphs in this formsince the number of points on the graph means that the memoryrequirement for storing all of the points is impractical. Therefore, inpractice, the sensor characteristic is approximated to a limited numberof points, or “voltage/flow pairs”, such that the values between thepoints have a linear relationship and can thus be calculated by means ofinterpolation. This type of interpolated graph is called a “data map”and an example of an interpolated map corresponding to the graph of FIG.4 is shown in FIG. 5. The map of FIG. 5 is defined by six voltage/flowpairs for simplicity although, in practice, such a map would containmany more voltage/flow pairs.

Such a two-dimensional map is most readily represented in the memory ofan engine management system by means of a one-dimensional map. It willbe appreciated that a one-dimensional map is readily expressed incomputer memory as a data structure known as a one-dimensional array, inwhich data is stored in a plurality of cells, each of which has adesignated index value. Such a one-dimensional map is also shown in FIG.5, each cell of which, in this case labelled I0 to I5, contains a valueof air mass flow that corresponds to a specified voltage value.

Through the use of a one-dimensional map, the sensor characteristicmodule 10 determines an interpolated value of air mass flowcorresponding to the received voltage signal. This value of air massflow will hereinafter be referred to as the measured value of air massflow, or “AMF_measured”.

Returning to FIG. 3, the filtered voltage from the air mass flow sensor4 is also input to the learn compensation module 12 of the air mass flowcalculation means 6. The learn compensation module 12 outputs acompensatory value of air mass flow which is dependent on the prevailingoperating point of the engine. The compensatory value of air mass flow,hereinafter referred to as “AMF_compensation”, is added to AMF_measuredat a summing junction 16. This results in a value of air mass flow beingoutput from the summing junction 16 which is corrected for sensor driftand provided as an input to the engine management system 8. Hereafter,the output of the summing junction 16 will be referred to as“AMF_corrected”.

In addition to providing a corrected value of air mass flow(AMF_corrected) to the engine management system 8, AMF_corrected alsoserves as a feedback signal that is input into the learning module 14 ofthe air mass flow calculation means 6. The learning module 14 performsthe function of updating the compensation values stored by the learncompensation module 12 so as to adapt continuously to any sensor driftthat may disrupt the accuracy of the air mass flow sensor 4. Thefunctionality of the learning module 14 will be described later.

As has been mentioned, the learn compensation module 12 calculates acompensatory output value of air mass flow which is dependent on thevalue of AMF_measured. As shown in FIG. 6, the learn compensation module12 includes a one-dimensional data map 13, hereafter referred to as a“compensation map”, having a plurality of cells, the number of cellscorresponding to the number of voltage/flow pairs by which the sensorcharacteristic is defined.

Each cell of the compensation map 13 contains a value that correspondsto the amount that the sensor characteristic is calculated to have“drifted” in the region of the sensor characteristic to which the cellapplies. The value in each cell is calculated and periodically updatedby the learning module 14, as will be described later.

Each cell is identified as having an index value which, in the exampleshown in FIG. 6, is numbered from I0 to I5. Each index value I0 to I5corresponds to voltage/flow pair points A to F, respectively. It will beunderstood that six index values are illustrated in this example forsimplicity whereas, in practice, many more index values may be used. Inorder to determine the contents of which cell should be output to thesumming junction 16, the learn compensation module 12 uses an inputvalue of AMF_measured and calculates a corresponding index value thatidentifies a particular cell of the compensation map 13. For example, ameasured voltage value V1 lies on the curve at point ‘G’ between thevoltage/flow pairs indicated by point ‘D’ and point ‘E’. The measuredvoltage V1 thus results in an interpolated value of AMF_measured of K1.The index attributed to point G is the index that corresponds to thevoltage/flow pair immediately preceding the measured voltage V1, i.e.the index I3 that is shown to the left of the value V1. Similarly, ifthe air mass flow sensor outputs a voltage of V2, which lies at point‘H’ between the voltage/flow pairs point ‘B’ and point ‘C’, thecorresponding index will be I1. Therefore, the value of AMF_compensationstored within the identified cell will be output to the summing junction16 and will be combined with the value of AMF_measured to provide acorrected value of AMF_corrected.

The index value that is identified in the above process is used insubsequent calculations in order for the learn module 14 to calculatethe appropriate compensation value to update the various cells of thecompensation map 13. Hereafter the index value shall be referred to as“AMF_index”.

Referring once again to FIG. 3, the learning module 14 receives inputvalues of AMF_measured and AMF_corrected. However, the learning module14 also receives a calculated value of air mass flow 20. The calculatedor “trusted” value of air mass flow 20 is a theoretical estimated valuethat is based on certain engine operating parameters and, in thisembodiment, is calculated as a function of engine speed, turbo boostpressure, turbo boost temperature, volumetric efficiency, swept enginevolume and the universal gas constant. It should be understood that theaforementioned engine parameters would be familiar to a skilled reader.

The trusted value of the air mass flow 20, hereafter referred to as“AMF_trusted”, may be trusted by the system as the true value of airmass flow at a particular engine operating point and, in thisembodiment, is calculated using the following equation:

${AMF\_ trusted} = {\left( \frac{{boost}\mspace{14mu}{pressure} \times {swept}\mspace{14mu}{volume} \times {volumetric}\mspace{14mu}{efficiency}}{{boost}\mspace{14mu}{temperature} \times {universal}\mspace{14mu}{gas}\mspace{14mu}{constant}} \right) \times {engine}\mspace{14mu}{speed}}$

The value of AMF_trusted is used by the learning module 14 to calculatethe amount by which the air mass flow sensor output has drifted basedupon the difference between the value of AMF_corrected and the value ofAMF_trusted. Based on the calculated difference, the learning module 14updates the cell of the compensation map 13 which corresponds to thecurrent operating point of the engine such that the effects of sensordrift are minimised. As a result, the value of AMF_corrected remainssubstantially equal to the value of AMF_trusted. The learning module 14is triggered to update the compensation map 13 only once certainenabling conditions are satisfied, as will now be described withreference to FIGS. 7, 8 and 9.

In FIG. 7, an initialisation step 700 represents the start of theprocess that is executed repeatedly in order to determine, or “search”for, the appropriate moment at which to initiate the learning module 14and, as a result, update the compensation map 13 stored by thecompensation module 12. The initialisation step 700 will be invokedeither when the engine is started or following a previous completeiteration of the process, as denoted by the input to the initialisationstep 700.

It should be mentioned at this point that modern diesel engine systemsemploy a technique typically referred to as Exhaust Gas Recirculation(EGR) to tackle the emission of nitrogen oxides in the exhaust gases ofthe vehicle. As would be well understood by a skilled reader, the EGRtechnique recirculates a portion of the exhaust gases back into the airintake of the engine. Since the recirculated exhaust gas contains arelatively low level of oxygen, the amount of oxygen entering theengine, and therefore the cylinders, is reduced which therefore reducescombustion temperatures. Although the use of EGR can have a minordetrimental effect on engine performance, the effect is generallyconsidered to be outweighed by the significant reduction of nitrogenoxide emissions that is achieved. Nevertheless, during the calculationof revised values stored in the compensation map 13 it is necessary thatEGR is inactive such that the mass of air flow entering the combustionchamber is equal to the mass of ambient air that is inducted into theengine. That is to say, the mass of airflow entering the combustioncylinders of the engine is equal to AMF_measured.

Returning to FIG. 7, step 700 represents the start of a first systemmode in which the system will perform engine monitoring prior toinitiating the functionality of the learning module 14. This system modewill hereafter be referred to as the “search mode”. The initialisationstep 700 serves to i) ensure that EGR does not remain disabled ii) reseta search timer, hereinafter referred to as “SEARCH TIMER”, and iii) readand store the current value of fuel demand.

The SEARCH TIMER is a means of checking how long the fuel demand hasbeen stable, i.e. has been fluctuating within predetermined limits ofthe previously stored fuel demand value.

At step 702, the SEARCH TIMER is incremented and at step 704 thestability of the fuel demand value is checked by determining if it iswithin a predetermined limit of the fuel demand value stored at step700. If fuel demand is not stable, the process flow will loop repeatedlythrough steps 706, 702 and 704 respectively until fuel demand isfluctuating within predetermined limits and thus is considered stable.At step 706, the SEARCH TIMER is reset to zero and the current value offuel demand is stored for comparison during the next loop through steps702 and 704.

Although not shown on the flow chart of FIG. 7, in addition to checkingthe stability of fuel demand the system will also continuously check aplurality of engine parameters that must be satisfied for the process toexecute. In the present embodiment the system also checks that: theengine brake is inactive; the exhaust brake is inactive; there are noair mass flow sensor faults; there are no boost air pressure sensorfaults; there are no boost air temperature sensor faults; the turbo isnot in an overspeed condition and the air compressor is inactive. Theaforementioned engine parameters would be familiar to the skilled readerand it should be understood that the system may be configured to checkmore or less parameters as desired. If the fuel demand is unstable, orany of the parameters referred to above are not satisfied, the systemwill not enter the search mode.

Once the enabling conditions are satisfied, it is checked at step 708that fuel demand has been stable for a sufficient period of time bycomparing the value of the SEARCH TIMER with a predetermined searchtimer limit. The process will loop through steps 708, 702 and 704 untilthe value of fuel demand has been stable for a sufficient period oftime.

If, as determined by steps 702, 704 and 708, the value of fuel demand isstable for a sufficient period of time such that the predetermined limitof the SEARCH TIMER is exceeded, the process proceeds to step 710 atwhich point a search counter, hereafter “SEARCH COUNTER”, isincremented.

The SEARCH COUNTER comprises multiple counter values, the number ofvalues corresponding to the number of cells in the compensation map 13.Each value in the SEARCH COUNTER has a corresponding predeterminedlimit, the purpose of which is to distinguish between commonlyencountered and rarely encountered engine operating conditions.

By way of explanation, consider engine operating points at which theengine typically spends a high proportion of its operating life, forexample running in a vehicle travelling at substantially constant andmoderate speed along a road having a substantially zero gradient. Sincethe engine is likely to reside at these operating points for arelatively large proportion of its life, it is not desirable to wastesystem resources by attempting to initiate the learning module on everyoccasion that these operating points arise. Thus, the values of theSEARCH COUNTER that relate to index values corresponding to commonlyencountered operating points e.g. index I2 on FIG. 6, have relativelyhigh limits. On the other hand, during such circumstances in which theengine is operating at rarely encountered operating points, for exampleat a very high engine load that would be required to propel a heavyvehicle up a high incline (e.g. index I5 on FIG. 6), it may be desirableto attempt to initiate the learning module 14 on every occasion thatthese circumstances arise (assuming that the fuel demand is stable andthe enabling conditions are met). Thus, the values of the SEARCH COUNTERthat relate to indices corresponding to rarely encountered operatingpoints may have relatively low limits. The predetermined limit values ofthe SEARCH COUNTER may be derived by experiment or, alternatively, bytheoretical means and may be configured upon installation of thesoftware embodying the invention into the appropriate data storage mediaof the vehicle.

Returning to FIG. 7, if it is determined at search counter check step712 that the relevant value of SEARCH COUNTER has not been exceeded, theprocess will return to step 706 at which point the SEARCH TIMER isreset, the last recorded value of fuel demand is stored and the checkfor fuel stability will repeat until such time as the relevant value ofSEARCH COUNTER has exceeded its predetermined limit. If it is determinedthat the relevant value of SEARCH COUNTER has exceeded its predeterminedlimit, the process will enter a second phase of operation, hereafterreferred to as the “learn mode”, at step 714.

The learn mode will now be described with reference to FIG. 8. When itis determined that SEARCH COUNTER has exceeded the predetermined limitfor a relevant cell, as described above with reference to FIG. 7, theprocess initiates the learn mode by deactivating EGR at step 714. Asdescribed previously, it is necessary for EGR to be deactivated toremove its contribution to the airflow into the cylinders of the engine.As a result of deactivating EGR, the value of AMF_trusted istheoretically equal to the value measured by the air mass flow sensor 4.

In addition, at step 714 a further timer will start counting, thepurpose of which is to record how long the process remains within thelearn mode. The further timer will hereafter be referred to as “LEARNDELAY TIMER” and is incremented whilst the process is in the learn mode.If at any point the count of the LEARN DELAY TIMER exceeds apredetermined value then the process will exit the learn mode and returnto the search mode at step 706 (also shown in FIG. 7) where the systemwill resume the search for enabling conditions. Since EGR has abeneficial effect on engine fuel economy and emissions, the LEARN DELAYTIMER ensures that EGR is not deactivated for longer than desired.

At step 804, another timer, hereafter referred to as “LEARN TIMER”, isinitialised to zero. In addition, the following values are recorded andstored: i) the current values of AMF_trusted as calculated by the AMFcalculation module; ii) the difference between AMF_trusted andAMF_corrected (hereafter referred to as “AMF_error”); and iii) the AMFindex value. The purpose of the LEARN TIMER is to ensure that the systemwaits until AMF_trusted, AMF_error and the AMF index value are stablefor a predetermined period of time prior to implementing the learningmodule 14, as will be described in detail later.

At steps 806 and 808, respectively, the process will check whether thefuel demand is stable and the count of the LEARN DELAY TIMER isexceeded. At this point, if fuel demand becomes unstable, the systemwill exit the learn mode and return to the search mode at step 706 (alsoshown in FIG. 7). Similarly, if the count of the LEARN DELAY TIMER isexceeded, the system will exit the learn mode and return to the searchmode at step 706.

However, if the LEARN DELAY TIMER is within its permissible limit, thecount of the LEARN DELAY TIMER and the count of the LEARN TIMER areincremented at step 810 and the process will proceed to check thestability of AMF_trusted as calculated by the AMF calculation module(step 812), the stability of AMF_error (step 814) and the stability ofthe index value (AMF_index, step 816) as determined by the learncompensation module 12.

If any of the aforementioned values of AMF_trusted, AMF_error andAMF_index are determined to be unstable, that is to say if any of thevalues are outside a predetermined limit when compared with thepreviously determined value of the parameter, the process will loop backto step 804, at which point the count of the LEARN TIMER will be resetto zero.

If, during steps 812, 814 and 816, the values of AMF_trusted, AMF_errorand AMF_index are determined to be stable, step 818 determines if theaforesaid parameters have been stable for a sufficient period of time byverifying whether the count of the LEARN TIMER has exceeded apredetermined limit. If not, the process loops to step 810, followingwhich steps 812, 814 and 816 will be repeated until such time as thecount of the LEARN TIMER has exceeded its predetermined limit.

The effect of the loop defined by steps 810 to 818 is therefore to judgehow long it is necessary for the values of AMF_trusted, AMF_error andAMF_index to be within predetermined limits before initiating thelearning module 14, for example, to allow the measured engine parametersto settle following deactivation of EGR. If all the aforementionedparameters are stable for a sufficient period of time, the learningmodule 14 will be implemented at step 820.

Referring now to FIG. 9, at step 902 the count of the SEARCH COUNTERthat corresponds to the current value of AMF_index will be reset tozero. It will be understood that if the SEARCH COUNTER was not reset atthis point, the process would not delay entering the learn mode if theenabling conditions are met during the next iteration through theprocess at the same value of AMF_index. A beneficial feature of theinvention, therefore, is that step 902 ensures system resources are notunduly wasted by initiating the learning module more often than isnecessary to update the compensation values stored by the learncompensation module 12.

At step 904, it is checked that the value of AMF_error is withinpredetermined maximum and minimum limits. Step 904 therefore ensuresthat the value of AMF_error has not changed significantly since it waslast measured at step 814, due to the onset of sudden erratic drivingconditions for example. If the value of AMF_error is outside the maximumand minimum limits, the system will not proceed to calculate revisedcompensation values. Instead, the system proceeds to step 906 where afurther check is carried out on the value of AMF_error. Step 906verifies whether or not the value of AMF_error merely represents anunstable value of AMF_error, such that meaningful learning data may notbe calculated, or if the air mass flow sensor is outputting faulty data.If the air mass flow is determined to be faulty, a sensor fault flagwill be set at step 908. The sensor fault flag is detectable by on-boardvehicle diagnostic routines and logged by the engine management system 8for the purposes of the next vehicle maintenance inspection. Inaddition, the fault flag may also trigger an audio/visual warning signalto the driver of the vehicle.

Following step 906, the process will return to the initialisation step700 (also shown in FIG. 7). Thus, the system exits the learn mode andbegins to search once again for enabling conditions.

If step 904 determines that the value of AMF_error is within acceptableerror limits, the learning module 14 will be initiated at step 910 inorder to calculate the compensation value that corresponds to thecurrent value of AMF_index at which the engine is operating. A globallyaccessible flag may be set at this point to indicate that leaning is tobe performed.

The learning process by which an appropriate cell within thecompensation map 13 is updated with a revised value will now bedescribed. Firstly, a gain factor is calculated, the purpose of which isto represent the magnitude of the difference between AMF_trusted andAMF_corrected. The gain factor (AMF_gain) is a function of the ratio ofthe trusted AMF to the corrected AMF, multiplied by a weighting factor.In this embodiment, the weighting factor may take two possible valueswhich are dependent on whether the value of AMF_corrected is greater orless than the value of AMF_trusted. In practice, the weighting factorthat is used will be derived empirically.

In this embodiment, the calculation of AMF_gain is expressed as:AMF_gain=1+AMF_weight×((AMF_trusted/AMF_corrected)−1).

As a result of the above expression, it will be understood that AMF_gainwill be unity if the value of AMF_corrected is equal to the calculatedvalue of AMF_trusted. Conversely, if the value of AMF_trusted does notequal AMF_corrected, AMF_gain will determine the responsiveness ofsubsequent calculations to drifting sensor signals.

The value of AMF_gain is then used to calculate the new value ofAMF_compensation that will overwrite the previously stored value ofAMF_compensation in the compensation map 13 within the cellcorresponding to the current value of AMF_index.

The new value of AMF_compensation for the current value of AMF_index maybe expressed as follows:AMF_Compensation[AMF_index]=AMF_gain[AMF_index]×(AMF_measured[AMF_index]+AMF_compensation[AMF_index])−AMF_measured[AMF_index]

Thus, from the above expression, it will be understood that ifAMF_corrected is equal to the calculated value of AMF_trusted (that isto say, if AMF_gain is unity), then the new value of AMF_compensationwill be equal to the old value of AMF_compensation. In other words, thenew compensation value will remain equal to the previously storedcompensation value if the AMF sensor output is not drifting.

Also, and preferably at the same time, the learning module 14 updatesthe value of AMF_compensation at the right-hand adjacent index as seenin FIG. 6. Thus, the calculation of the value of AMF_compensation at theadjacent index point in the compensation map 13 may be expressed as:AMF_Compensation[AMF_index+1]=AMF_gain×(AMF_measured[AMF_index+1]+AMF_compensation[AMF_index+1])−AMF_measured[AMF_index+1]

The abovedescribed refinement therefore takes account of the fact thatthe value of AMF_measured is most likely to lie between two voltage/flowpairs. Referring to FIG. 6 as an example, point I lies between points Cand D and, as has been described above, the calculations relating toAMF_compensation will relate to index I2 and I3 and the correspondingvalues in the compensation map will be updated.

As a further refinement, the system may also be configured to update thevalue of the compensation map at preferably two secondary adjacent indexpoints. However, a modified value of AMF_gain is utilised for thiscalculation which is referred to as AMF_gain_spread. The use ofAMF_gain_spread is of particular importance when the compensation map isbeing updated at index values corresponding to engine operating pointsthat occur relatively infrequently. The updated compensation map valueat a particular index therefore influences the compensation values atadjacent cells and the benefit of carrying out the learning process atone identified index will be distributed or ‘spread’ to adjacent cells.Referring once again to FIG. 6 as an example, although the compensationvalues in the compensation map 13 corresponding to indices I2 and I3 areupdated as described above, the compensation values at indices I1 and I4may also be updated, but by a lesser amount than the compensation mapvalues at indices I2 and I3. Once the relevant values of thecompensation map have been updated at step 910, the process returns toinitialisation step 700.

Having described various embodiments of the invention, it will beunderstood by those who practice the invention, and those skilled in theart, that various modifications and improvements may be made to theinvention without departing from the scope of the invention as definedby the claims. For example, it should be appreciated that the variouspredetermined limits referred to herein are application specificprogrammable variables that may be programmed at manufacture or uponinstallation into an engine controller or the appropriate controlsoftware embodying the invention. Such predetermined limits may becalculated by theoretical or empirical means. Furthermore, although theabove invention has been described with reference to functional modulesfor the sake of clarity, i.e. learn compensation module 12 and learningmodule 14, it will be appreciated that it is not essential for thefunctionality of the invention to be embodied in this modular mannerwhen the invention is implemented in on-board vehicle software.

1. A method of compensating for the error of a signal indicative of themass of air flow from an air mass flow sensor for an internal combustionengine, the method comprising: deriving an air mass flow value from anair mass flow sensor output; calculating a compensated air mass flowvalue based on the air mass flow value and a compensation value;calculating an estimated air mass flow value from engine operatingparameter values; calculating a new compensation value based upon thecompensated air mass flow value and the estimated air mass flow value;and using the new compensation value for subsequent calculations of thecompensated air mass flow value; wherein the step of calculating a newcompensation value is triggered by a timer having a plurality ofpredetermined limits.
 2. The method as claimed in claim 1, wherein theair mass flow sensor has a corresponding interpolated data maprepresenting air mass flow as a function of air mass flow sensor output,the air mass flow value being derived from the interpolated data map inresponse to the air mass flow sensor output.
 3. The method as claimed inclaim 2, wherein the interpolated data map is defined by a plurality ofvoltage/flow pairs.
 4. The method as claimed in claim 3, wherein each ofthe plurality of voltage/flow pairs is identified by a respective indexvalue.
 5. The method as claimed in claim 4, wherein each index valuerelates to a predetermined engine operating region.
 6. A method ofcompensating for the error of a signal indicative of the mass of airflow from an air mass flow sensor for an internal combustion engine, themethod comprising: deriving an air mass flow value from an air mass flowsensor output; calculating a compensated air mass flow value based onthe air mass flow value and a compensation value; calculating anestimated air mass flow value from engine operating parameter values;calculating a new compensation value based upon the compensated air massflow value and the estimated air mass flow value; and using the newcompensation value for subsequent calculations of the compensated airmass flow value; wherein the air mass flow sensor has a correspondinginterpolated data map representing air mass flow as a function of airmass flow sensor output, the air mass flow value being derived from theinterpolated data man in response to the air mass flow sensor output;wherein the interpolated data map is defined by a plurality ofvoltage/flow pairs; wherein each of the plurality of voltage/flow pairsis identified by a respective index value; wherein each index valuerelates to a predetermined engine operating region; wherein thecompensation value is selected from a plurality of compensation values,the number of compensation values corresponding to the number of indexvalues, and wherein the compensation value is selected based upon theindex that is identified according to the current engine operatingregion.
 7. The method as claimed in claim 6, wherein the plurality ofcompensation values are stored in the form of a one-dimensional datamap.
 8. A method of compensating for the error of a signal indicative ofthe mass of air flow from an air mass flow sensor for an internalcombustion engine, the method comprising: deriving an air mass flowvalue from an air mass flow sensor output; calculating a compensated airmass flow value based on the air mass flow value and a compensationvalue; calculating an estimated air mass flow value from engineoperating parameter values; calculating a new compensation value basedupon the compensated air mass flow value and the estimated air mass flowvalue; and using the new compensation value for subsequent calculationsof the compensated air mass flow value; wherein the air mass flow sensorhas a corresponding interpolated data map representing air mass flow asa function of air mass flow sensor output, the air mass flow value beingderived from the interpolated data map in response to the air mass flowsensor output; wherein the interpolated data map is defined by aplurality of voltage/flow pairs; wherein each of the plurality ofvoltage/flow pairs is identified by a respective index value; andwherein the step of calculating a new compensation value is triggered bya timer having a plurality of predetermined limits.
 9. The method asclaimed in claim 8, including incrementing the timer to count the numberof times it is verified that each one of a plurality of enablingconditions is satisfied, wherein the step of triggering the calculationof a new compensation value is initiated when the number of timesexceeds the predetermined limit for the associated engine operatingregion.
 10. The method as claimed in claim 9, wherein the timer limitsare selected to be relatively high for frequently encountered engineoperating regions and relatively low for engine operating regions thatare encountered infrequently.
 11. The method as claimed in claim 9,wherein the step of verifying the plurality of enabling conditionsincludes comparing a first stored fuel demand value with a second storedfuel demand value and determining if the difference between the firstand second stored fuel demand values is within a predetermined limit.12. A method of compensating for the error of a signal indicative of themass of air flow from an air mass flow sensor for an internal combustionengine, the method comprising: deriving an air mass flow value from anair mass flow sensor output; calculating a compensated air mass flowvalue based on the air mass flow value and a selected one of a pluralityof compensation values, wherein each of the plurality of compensationvalues corresponds to a predetermined engine operating region;calculating an estimated air mass flow value from engine operatingparameter values; calculating a new compensation value based upon thecompensated air mass flow value and the estimated air mass flow valueand updating a selected one of the plurality of compensation values withthe new compensation value; and using the new compensation value forsubsequent calculations of the compensated air mass flow value; whereinthe step of calculating a new compensation value is triggered by a timerhaving a plurality of predetermined limits.
 13. The method as claimed inclaim 12, wherein the plurality of compensation values are stored in theform of a one-dimensional data map.
 14. The method as claimed in claim12, wherein the air mass flow sensor has a corresponding interpolateddata map representing air mass flow as a function of air mass flowsensor output, the air mass flow value being derived from theinterpolated data map in response to the air mass flow sensor output.15. The method as claimed in claim 14, wherein the interpolated data mapis defined by a plurality of voltage/flow pairs.
 16. The method asclaimed in claim 15, wherein each of the plurality of voltage/flow pairsis identified by a respective index value that relates to apredetermined engine operating region.
 17. The method as claimed inclaim 12, including incrementing the timer to count the number of timesit is verified that each one of a plurality of enabling conditions issatisfied, wherein the step of triggering the calculation of a newcompensation value is initiated when the number of times exceeds thepredetermined limit for the associated engine operating region.
 18. Themethod as claimed in claim 17, wherein the timer limits are selected tobe relatively high for frequently encountered engine operating regionsand relatively low for engine operating regions that are encounteredinfrequently.
 19. An engine control system for use in an internalcombustion engine, the engine control system including a compensatingunit for compensating for the error of a signal indicative of the massof air flow output from an air mass flow sensor for an internalcombustion engine, the compensating unit including: a deriving unit forderiving an air mass flow value from an air mass flow sensor output; asumming unit for calculating a compensated air mass flow value based onthe air mass flow value and a compensation value; a calculating unit forcalculating an estimated air mass flow value from engine operatingparameter values; and a learning unit for calculating a new compensationvalue based upon the compensated air mass flow value and the estimatedair mass flow value and using the new compensation value for subsequentcalculations of the compensated air mass flow value; and a timer fortriggering the learning unit to calculate the new compensation value,the timer having a plurality of predetermined limits.